Transfer factor is a dialyzable extract of lymphocytes from human peripheral blood taken from subjects with cellular immunity against a known battery of antigens. It was first reported as being able to transfer delayed type hypersensitivity to ubiquitous antigens, and later was used therapeutically to correct immunodeficiency states. Its chemical characteristics are unknown, its mechanism of action is unknown, and the exact specificity requirements are unclear. It does increase antigen specific cellular immunity concomitant with reported clinical improvement in subjects in which it has been used, but it is not known if these two phenomena are related. The project described here proposes to use an in vitro system to investigate human transfer factor as to its specificity requirements, mechanism of action, and chemical nature.